This invention generally relates to weight scales and, more particularly, to electronic weight scales and strain gauge assemblies used therein.
Electronic weight scales are well known which employ strain gauges that are attached to a flexible member. Compression or stretching of the strain gauge during flexing of the member causes it to change resistance which is related to the load, or weight, causing the member to flex. This resistance change is then sensed and applied to an LED or other suitable display for visually indicating the weight placed on the scale.
In particular, it is known to employ an elongate, flexible, cantilevered beam which is tightly secured to a rigid frame at one end and which receives a load intermediate the secured end and its distal end that causes it to flex and thereby energize a strain gauge attached thereto. In order to prevent excessive flexing, it is often necessary to provide the flexible, cantilevered beam with a substantial thickness which prevents incorporating it in a scale of desirable low profile. Customarily, the load is applied to the beam at a single fixed point spaced a substantial relative distance from the fixed end which defines a single movement arm. The precise length of this movement arm is critical for accurate measurement but is difficult to achieve. In addition, non-linearity in the relationship between the load and amount of resultant flexing is caused by the flexing. As the beam flexes more, an increasing large component of the load force becomes directed along the length of the beam instead of transverse to the beam. Further, since the force is applied to only one point on the beam, uniform distribution of the load to this one point is difficult to achieve.
An electronic scale is known which alleviates some of these problems but which has other disadvantages. In this electronic scale, the ends of an elongate flexible member with a strain gauge are torqued, or rotated, in opposite directions to cause the intermediate portion of an elongate flexible member to flex downwardly in response to a load. The ends pass through slots through intermediate portions of elongate axis in response to a load. These arms are supported on one side by knife edge supports and receive loads at their other sides through knife edge load transmission members to cause them to twist and thus apply a torque to the ends of the flexible member at the edges of the slots.
This approach may enable a lower profile than the cantilevered structure noted above and achieves a certain degree of load distribution by virtue of the spaced, elongate arms. However, position of the flexible member relative to the edges of the slots in the arms is not fixed and nonlinearities are still introduced. Such nonlinearities are introduced in part because of high friction sliding movement between the arms and the flexible member which resist flexing to an increasing degree with increasing loads. In addition, the flexing of the member causes apparent foreshortening between the load points which introduces further nonlinearities.
A further problem with known weight scales is that only the scale housing protectively encloses the relatively fragile strain gauges. Accordingly, the strain gauges are relatively unprotected prior to assembly or during repair when the housing is removed. In addition, known scales are constructed in a way which does not facilitate substantial subassembly of modular units and the cost reducing advantages resultant therefrom.
Also, these prior weight scales use a combination of flexible members, underlying support members and overlying force transmitting members that, although relatively simple and inexpensive, could be better arranged to reduce manufacturing and assembly costs. For example, U.S. Pat. No. 4,457,385 to Provi discloses four bell crank assemblies to transmit the weight of a person to its sole load sensing beam that flexes horizontally. U.S. Pat. No. 2,899,191 to Hunt discloses a pair of spaced, smooth beam members that flex vertically. The smooth beams rest on four underlying fulcrum edges and the user's weight is transmitted to the beams by journalled lugs.